Prior radio frequency (RF) bipolar technology devices utilize common base wells containing multiple emitter finger device structures. The frequency response of such devices is lithographically established by the emitter width. RF power gain, distortion figures, noise factor and efficiency are significantly impacted by the magnitude and linearity of the associated parasitic base-to-collector capacitance, base-to-emitter capacitance, and base resistance. These prior RF devices have excessive, nonlinear extrinsic base-to-collector capacitance and base resistance. To attain a very high speed for a junction bipolar transistor, it is necessary to diminish the base resistance, base-to-emitter capacitance and base-to-collector capacitance.
A common process for forming bipolar transistors includes the steps of doping an n-type silicon substrate layer that acts as a collector terminal with p-type dopant to form a base region. A layer of polysilicon is formed on the surface of the substrate layer to provide electrical contact to an emitter region and to the base region. The emitter region is formed by diffusing an n-type dopant from the layer of polysilicon into the base region in the substrate layer.
To enhance electrical contact to the base region, an additional base contact region is formed by diffusing a p-type dopant from the layer of polysilicon into the base region in the substrate layer. However, during subsequent high temperature processing, some of the p-type dopant used to form the base contact region migrates through the layer of polysilicon and gathers in the layer of polysilicon in the n-type emitter portion of the bipolar transistor. The lateral diffusion of the p-type dopant forms a minority carrier concentration gradient in the layer of polysilicon above the emitter region. The presence of this concentration gradient creates variability in the resistance of the emitter portion of the layer of polysilicon. This variability in resistance makes it more difficult to predict and control the exact performance characteristics of the bipolar transistor.
Conventional methods to compensate for this problem are directed towards increasing the gain (beta) of the bipolar transistor and reducing the breakdown voltage of the transistor. However, these solutions are not applicable when the bipolar transistor is intended for use in RF power applications. RF power applications require relatively low beta values with high breakdown voltage and high current carrying capability.
Accordingly, a need exists for a bipolar transistor for RF and other bipolar and/or MOS device applications requiring very high performance. It is desired for the device to have very high frequency, linear, rugged, low noise performance in high speed and/or high speed/high power communication applications and other applications requiring high speed and high frequency performance. It is also desired that inherent bipolar device parasitics be reduced to near theoretical minimums resulting in low noise and distortion products, maximum efficiencies, high linearity, and high power gains.
It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for clarity.